Organic electroluminescent devices (hereinafter referred to as organic EL devices) are expected as new self-light emitting devices. An organic EL device has a stacked layered structure that a carrier transport layer (an electron transport layer or a hole transport layer) and a luminescent layer are formed between a hole injection electrode and an electron injection electrode.
Electrode materials having a large work function such as gold or ITO (indium-tin oxide) are employed for the hole injection electrode, while those having a small work function such as Mg (magnesium) or Li (lithium) are employed for the electron injection electrode.
Organic materials are employed for the hole transport layer, the luminescent layer and the electron transport layer. Materials having the property of a p-type semiconductor are employed for the hole transport layer, while those having the property of an n-type semiconductor are employed for the electron transport layer. The luminescent layer is also composed of organic materials that have carrier transportability such as electron transportability or hole transportability and emit fluorescence or phosphorescence.
These hole injection electrode, hole transport layer, luminescent layer, electron transport layer and electron injection electrode are stacked in turn to form the organic EL device.
Each function layer such as the hole transport layer, the electron transport layer and the luminescent layer is constituted by a plurality of layers or omitted depending on the organic materials to be used.
In such an elementary structure as shown in Appl. Phys. Lett., Vol. 55, pp. 1489-1491 by Chihaya Adachi et al., for example, only two organic layers, which are a luminescent layer and an electron transport layer exist between a hole injection electrode and an electron injection electrode. This is because the luminescent layer composed of luminescent materials called NSD has excellent hole transportability and hence serves also as a hole transport layer.
Further, the elementary structure shown in Appl. Phys. Lett., Vol. 51, pp. 913-915 (1987) by C. W. Tang et al. is constituted by two organic layers, which are a hole transport layer and a luminescent layer. In this case, tris(8-hydroxyquinolinato)aluminum (hereinafter referred to as Alq) contained in the luminescent layer serves to both emit light and transport electrons.
On the other hand, the elementary structure shown in Appl. Phys. Lett., Vol. 69, pp. 2160-2162(1996) by S. A. Van Slyke et al. is constituted by three organic layers, which are a hole injection layer, a hole transport layer and a luminescent layer. In this case, the hole injection layer is composed of copper phthalocyanine, serving for the same function as the hole transport layer, which results in two hole transport layers existing in the entire device.
Thus, the number of the electron transport layer, hole transport layer and luminescent layer can freely be adjusted depending on the organic materials to be used.
In the organic EL devices, visible light of blue through red can be obtained by selecting the organic materials constituting the luminescent layers. Accordingly, a full-color display can be realized by use of organic EL devices that emit respective monochromatic lights of red, green and blue which are three primary colors (RGB) of light.
In red light, green light and blue light obtained from the organic EL devices, the green and blue lights are stable light. In contrast, as for red through orange light, i.e., red-orange light, it is difficult to obtain the light with high luminance and high luminous efficiency. This is because there exist no solid organic materials that emit fluorescence or phosphorescence of red to orange at high efficiency.
For example, as the organic materials for the luminescent layers of the organic EL devices that emit red-orange light, DCM-based materials being laser dye-based materials such as 4-(dicyanomethylene)-2-methyl-6-julodin-4-yl-vinyl)-4H-pyran (hereinafter referred to as DCM) and the like that has such a structure as represented mainly by a formula (10) shown below are employed. In such organic EL devices employing the DCM-based materials, however, luminous efficiency can hardly be increased.
